The invention relates generally to solid state lasers and more particularly to solid state laser resonator configurations which oscillate to spherical modes.
The standard (Schawlow-Townes) laser configuration includes an active medium placed between a pair of spaced mirrors which define a resonant cavity with cylindrical symmetry. The modes of oscillation of this cylindrical resonator have Gaussian intensity profiles and behave essentially as plane waves.
However, the concept of a resonator can be extended to any fully bounded surface in which standing waves are created. Theoretical calculations of spherical modes are described in Stratton, Electromagnetic Theory, McGraw Hill, 1941, P. 554. Although no practical laser configuration with a spherical resonator has been demonstrated, a few researchers have performed preliminary experiments that show the existence of spherical modes in a nonstandard resonator configuration.
Garrett et.al., "Stimulated Emission Into Optical Whispering Modes of Spheres," Phys. Rev., V. 124, N. 6, P. 1807, Dec. 15, 1961, shows stimulated emission into optical whispering modes of a spherical sample of Sm-doped CaF.sub.2. Spheres of 1-2 mm diameter were placed in a liquid hydrogen Dewar and pumped by flashlamps.
Lin et.al., "Some Characteristics of a Droplet Whispering-Gallery Mode Laser," Opt. Let., V. 11, N. 10, P. 614, Oct. 1986, and Tzeng et.al., "Laser Emission from Individual Droplets at Wavelengths Corresponding to Morphology-Dependent Resonances," Opt. Let., V. 9, N. 11, P. 499, Nov. 1984, observed laser emissions from 40-60 micron liquid dye solution droplets pumped by Q-switched or CW lasers. Multiple frequency outputs are produced. Snow et.al., "Nonlinear Optics with a Micrometer-Size Droplet," Optics News, P. 5, May 1986, describes amplified Raman waves travelling around the circumference of a droplet as well as lasing from droplets.
Some spherical modes may occur even in an conventional cylindrical resonator as undesired parasitic modes. Linn et.al., "Effect of Trapped Light on the Output of a Ruby Laser," App. Opt., V. 4, N. 9, P. 1099, Sept. 1965, describes the near field patterns from a flashlamp pumped ruby rod (in a conventional resonator) due to the action of trapped modes, including modes caused by total internal reflection from the ruby walls; however, these modes are spurious and transitory, and cannot become permanently established. Yajima et.al., "Cylindrical Mode of Oscillators in a Ruby Optical Maser," Symposium on Optical Masers, P. 111, Apr. 1963, modified a parallel-plate resonator by forming a hole in one of the mirrors to suppress the normal high gain modes of oscillation through the center of a ruby rod and produce a helical mode of oscillation in an annular ring of the rod. U.S. Pat. No. 3,440,561 shows a conventional resonator configuration in which the gain medium is a sphere but the resonator is defined by external mirrors; only a cylindrical portion of the sphere is used as the active gain medium as in a conventional rod amplifier with the spherical shape of the gain medium being used as a lens to allow switching between different resonator defining external mirrors.
Thus while some spherical modes of oscillation have been experimentally observed as an optical effect in a material, the prior art does not disclose a practical laser which uses a spherical resonator configuration to generate a useful output in the form of spherical waves. It is necessary and desirable to provide a practical laser design which uses a solid state laser gain medium such as Nd:YAG, which has a simple and efficient pump source such as a laser diode, which defines a spherical resonator cavity, which allows effective matching of the pumping region to the active gain medium of the resonator, which produces an output having useful and controllable characteristics such as single frequency and single longitudinal mode, and which has effective means for output coupling. A laser with these features would produce an output which would be useful in a variety of applications, including tracking, levelling, holography, and ring laser gyroscopes.